Various types of biometric systems are used more and more in order to provide for increased security and/or enhanced user convenience.
In particular, fingerprint sensing systems have been adopted in, for example, consumer electronic devices, thanks to their small form factor, high performance, and user acceptance.
Among the various available fingerprint sensing principles (such as capacitive, optical, thermal etc), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important issues.
All capacitive fingerprint sensors provide a measure indicative of the capacitance between each of several sensing structures and a finger placed on or moved across the surface of the fingerprint sensor.
With the increasing and more varied usage of fingerprint sensors come requirements for increased resolution of the fingerprint sensor. In some applications it may desirable to provide fingerprint sensors having a better image quality offering resolution higher than the standard 500 dpi. Image resolution and quality will have a positive effect on false rejection rate (FFR) and false acceptance rate (FAR) performance and could be used as input for fast finger detect. It could also enable smaller and thereby more economical area sensors.
However, some devices applications require high resolution and some do not, the required resolution may also differ between different use cases. Accordingly, it is desirable to provide a fingerprint sensor offering a high resolution but where the resolution can be controlled to accommodate different needs.
In general obtaining a controllable (flexible) resolution is not as straightforward as it may seem. The size of the sensing structures (pixels) and the local amplifiers are mostly fixed and a different resolution means a full redesign of the sensor which doesn't address the different needs for different markets and customers. Also, merely scaling up the resolution and measuring methods could end up in unrealistic component density in the imaging matrix, either leading to the need of smaller and more expensive process nodes or to lower yield.
Accordingly, there is a need for an improved fingerprint sensor which is usable in a wide range of applications.